Hydraulic shock absorbers or suspension dampers are used on vehicles to dampen oscillations caused by uneven road surfaces or vehicle accelerations. Hydraulic dampers absorb vibrational energy using hydraulic principles. Dampers employ a piston within a cylinder, where the damper extends and retracts (compresses) in response to vehicle movements. Fluid is forced through a series of orifices and relief and check valves in order to control the motion of the piston and achieve damping.
Unfortunately, however, such dampers are passive in that the orifices and the relief and check valves produce the same damping characteristics for all road surfaces. Dampers should optimally be able to assume different operating characteristics under varying operating conditions. To correct this shortcoming, vehicle suspension systems have incorporated selectable or adaptive control systems. In these systems, the vehicle operator is able to select between firm or soft suspension ride modes. A small direct current motor within the damper opens or closes a valve which operates in parallel with the standard orifices and relief valves within the damper. As a result, the vehicle operator may modify the vehicle ride mode to accommodate personal preference.
Adaptive control is limited, however, because the parallel valving must be set in either a fully on or a fully off position. It would be desirable for the control system to be proportionally controlled, so that the characteristics of the damper could change in relation to real-time varying road conditions. It may also be desirable for the control system to incorporate other factors which may affect the optimal suspension ride mode, such as vehicle speed, load, or the present position of the damper. Additionally, adaptive control systems typically require 0.25 seconds or more for the selected ride mode to take effect. Even if the operator desired to modify the ride mode for a change in road conditions, the selectable system may not be able to respond in time. For example, if the system was set for a soft ride, it may be desired to quickly change it to a firm ride, for example to increase the tire-to-road force when going over a series of bumps so as to improve turning ability or to negotiate a turn so as to prevent the vehicle from rolling.
It is also desirable that a proportional control valve revert to an intermediate setting should a failure, such as a loss of power, occur. Reverting to a firm ride may result in discomfort to the vehicle occupants and reverting to a soft ride may result in reduced control over the vehicle. Moreover, it is desired that a proportional control valve be energy efficient, and consume as little electrical power in operation to produce a given suspension mode as possible.